You might not have heard of Operation Perdition - but it remains one of the biggest and most disturbing child abuse cases in British history.

It cracked a paedophile ring which targeted more than 18,000 children across six continents - with the majority from Britain - grooming them into performing lewd sexual acts online.

The network posed as a young girl to manipulate youngsters into sharing pictures and videos, before blackmailing them to perform more and more.

Some were even forced to involve their friends and siblings for the twisted entertainment of their tormentors.

Now, the sickening story of how the abuse unfolded is the focus of a chilling new TV documentary called Hunting the Paedophiles: Inside the National Crime Agency, which airs at 10pm tonight on Really.

The episode reveals how officers were first alerted to the gang's tactics in November 2011.

Surrey-based Detective Constable Matt Strickland was called in to interview a 13-year-old boy who had started chatting to who he thought was a 14-year-old girl from Italy called 'Justina'.

This had been going on for a couple of months; she said she liked English boys, having ‘a chat and a laugh’ with them.

But eventually the boy had been asked to expose different parts of his body and do indecent acts via video camera. Things had then taken a truly dark turn.

The victim's devastated mother explains. "She was asking him to do more and more bizarre things that he really didn't feel comfortable with.

"He realised it was serious and that this person needed to be stopped."

But unfortunately, the family's torture wasn't to end there.

The 'girl' soon shared explicit footage of the boy performing sex acts with all of the pupils at his school - and openly revelled in the pain the post caused, writing "r u scared..." and "does it hurt?"

Police moved quickly to launch a major investigation and soon uncovered cases of the same offender exploiting children as young as 10 across 34 different counties - with experts describing the harrowing demands sent to victims as being "dripping with hate".

The severity of the emerging case meant it was soon escalated from Surrey Police to the NCA and its CEOP (Child Exploitation and Online Protection) command unit.

Reports of similar abuse would eventually spread to every single county in the UK.

“They have been tricked,” says DC Strickland, speaking of how the victims were groomed.

“This person they thought was a friend is not a friend at all. This person actually only wanted one thing and now they have got it.”

The documentary - the first of an eye-opening two-part series on Really, the second of which airs on October 2 at 10pm and looks at how the NCA are tackling Britain's £100billon money laundering industry - speaks to the team at the heart of the probe to reveal how they finally cracked the case.

Viewers learn how officers in the UK tracked the user's IP addresses and trawled profiles online to eventually pinpoint his computer as being over 4000 miles away in Manama, Bahrain.

But after they dug deeper, investigators discovered a network of paedophiles - all aged under 21 and living with their families - colluding with each other to run multiple abuse scams under the same username.

Officers worked with local police to swoop on their suspects and searches of their hard drives uncovered more than 30,000 indecent images of underage boys.

The four-man gang, below, were eventually charged with child sex offences against children on six different continents - with most of their victims in the UK.

In 2014 they were sentenced to a combined total of 20 years, while officers also went on to uncover similar crimes by a British man who was later jailed for four years.